where \(E_{el}\)  can be computed with standard electronic structure methods[1]}, [2]}. It is important to note that the summation over the cells of the molecular lattice (index \(l\)  in Eq. REF ) is in principles extended to an infinite number of them. However, the forces among two atoms decay with their distance and the elements of \(\Phi \)  will decays to zero for distant enough lattice cells. It is then possible to estimate all the elements of \(\Phi \)  by performing electronic structure simulations on a small supercell, i.e. the replication in space of the crystal unit-cell[3]}. The size of the supercell actually needed to converge the elements of \(\Phi \)  should determined case-by-case, depending on the size of the unit-cell and the nature of atomic interactions. For instance, large unit-cells with only short-range interactions will necessitate of only small supercells, such as a \(2\times 2\times 2\)  repetition of the unit-cell along the three different crystallographic direction. Polar crystals usually do not fall in this category due to the long range nature of dipolar electrostatic interactions, but dedicated expressions to account for this effect fortunately exist[4]}.
